Heat the olive oil in a skillet over medium-high heat. Sear the brisket on all sides until browned, about 3-4 minutes per side. Remove from skillet and set aside.
In the same skillet, add the sliced onion and garlic. Sauté until the onion is translucent, about 5 minutes.
In the slow cooker, combine the beef broth, barbecue sauce, brown sugar, Worcestershire sauce, smoked paprika, salt, black pepper, and cayenne pepper. Stir to combine.
Place the seared brisket on top of the sauce mixture in the slow cooker. Add the sautéed onions and garlic on top of the brisket.
Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the brisket is tender and easily shredded with a fork.
Once cooked, remove the brisket from the slow cooker and let it rest for 10 minutes before slicing. Serve with additional barbecue sauce if desired.